US3203792A

US3203792A - Highly corrosion resistant nickel-chromium-molybdenum alloy with improved resistance o intergranular corrosion

Info

Publication number: US3203792A
Application number: US393827A
Authority: US
Inventors: Scheil Margarete; Class Immanuel; Graefen Hubert
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 1961-04-01
Filing date: 1964-08-31
Publication date: 1965-08-31
Anticipated expiration: 1982-08-31
Also published as: GB956166A; JPS495812B1; DE1210566B

Description

Aug. 31, 1965 E. scHEn. ETAL 3,203,792

HIGHLY CORROSION RESISTANT NICKEL-CHROMIUM-MOLYBDENUM ALLOY WITH IMPROVED RESISTANCE TO INTERGRANULAR CORROSION Filed Aug. 5l. 1964 en A FI G. 3

linear decrease inthickness m /year o 2o 4e|6IQ 8o |00 percent by weight of HCOOH (boiling point) thickness of alloy sheeting in mm lNvENToRs: ERicH scHEn. DEcEAsED BY MARGARET scHE||.,LEGAL REPRESENT'ATIVE IMMANUEL cLAss HUBERT @RAI-:PEN

Si content in percent ATT'YS United States Patent O 3,203,792 HIGHLY CORRG'SIN RESSTANT NTCKEL- CHROMIUM-MGLYBDENUM ALLOY WITH fliMlRGVED RESISTANCE T INTERGRAN- ULAR CORROSHBN Erich Scheil, deceased, late of Stuttgart, Germany, by Margarete Scheil, heir and legal representative of minor heirs, Stuttgart, Germany, and Immanuel Class and Hubert Graefen, Ludwigshafen, Rhineland, Germany, assignors to Badische Anilin- & Soda-Fabrik Aktiengesellschaft, Ludwigshafen, Rhineland, Germany `Filed Aug. '31, 1964, Ser. No. 393,827 Claims priority, application Germany, 'Api'. '1, 1961,

4 claims. (l. *7s-171) 54 to 60% nickel, 14.5 to 16.5% chromium, 15 to 17% molybdenum, 4 to 7% iron and a maximum of 0.1% carbon and of 1% silicon.

If desired, the alloy can also contain from 3 to 4.5%

ltungsten and up to 3% cobalt.

Alloys with the above mentioned composition, however, are heterogeneous in the state of equilibrium and, therefore, have low corrosion resistance. To impart to them favorable structural characteristics they must be subjected to a heat treatment which consists of annealing at a temperature above 1,200 C. with a subsequent water quench. Upon further heating to 600 to 1,100o C. the supersaturation produced at room temperature in the jstructure brings about the formation of new phases, which preferentially .appear at the grain boundaries. These types of compounds are rich in chromium and molybdenum and are designated as sigmaphase. However,

Through the impoverishment of the grain boundary areas in chromium and molybdenum, these alloys show high susceptibility to intergranular corrosion, whereas the precipitates themselves have reduced workability as a result, which can cause complete embrittlement of the material. This high susceptibility to grain decomposition with reduced toughness in the heat-affected zone also appears after welding and can only be eliminated by an additional solution annealing and water quench treatment. This procedure, however, is technically not always feasible 'because of the size of certain of the equipment and the dif- 'iiculties involved in annealing and quenching the product,

especially because of the high temperature which has to be employed. For these reasons, nickel-chromium-molybdenum alloys cannot be used in many instances.

It is an object of the present invention to provide -nickel-chromium-molybdenum alloys with improved resistance to intergranular corrosion.

Another object of the invention is -to provide alloys `which when used for the manufacture of apparatus need not be subjected to a heat treatment after welding.

ln general, the present invention comprises the discovery that a greatly improved nickel-chromium-molybdenum alloy can be produced if the silicon content of the alloy is maintained at an extremely low level. In all Vother phases with a similar composition are also formed.

for solution annealing. Vbut silicon-free alloy (nickel 58.8%, chromium 15.5%,

3,203,792 Patented Aug. 3l, 1,965

ICC

events, the silicon level should be less than 0.2%, and

preferably should be from 0 to 0.1%, and more preferably from 0 to 0.012%. Ordinarily, the above described nickel-chromiummolybdenum alloys have a silicon content of 0.5 to 1%. It has been suggested in the past that the presence of silicon is needed for deoxidation of the alloy and to improve the forgeability of the alloys. Silicon is also almost always incorporated in the alloy as an impurity due to the fact that silicon-containing materials are used as deoxidation agents.

It has been found that by maintaining the silicon content of the nickel-chromium-molybdenum alloy at an extremely low level the precipitation rate of the phases mentioned above, and especially the -sigma-phase, is substantially decreased. Likewise, the solubility of chromium and molybdenum in the nickel-rich matrix is increased so that lower temperatures are needed for solution annealing. Likewise, alloys containing from 0 to 0.1% silicon, in most cases, especially if the thickness does not exceed 10 mm., need not be subjected to a further heat treatment after welding.

According to the invention, a highly corrosion and heat resistant nickel-chromium-molybdenum alloy is obtained with improved resistance to intergranular corrosion by using: a nickel content of 40 to 65%, preferably of 55 to 60%, part of the nickel, up to a maximum of 20%, if desired being replaced by cobalt; a chrome content of 14 to 26%, preferably 22 to 25%; a molybdenum content of 3 to 18%, preferably 14 to 17%; an iron content of O to 30%, preferably 0 to 7%; a tungsten content of 0 to 5%; a carbon content of not more than 0.1%; a manganese content of up to 3%; a silicon content of from 0 to less than 0.2%; as well as a phosphorus and sulfur content totaling not more than 0.1%, if these are produced from corresponding metals or master alloys which are free of silicon and by subsequent deoxidation with a silicon-free alkaline-earth metal, preferably magnesium, or a silicon-free alkaline-earth metal master Ialloy, prefererably a silicon-free nickel-alkaline-earth metal master alloy, or with a silicon-free titaniuml master alloy.

According to the invention, instead of silicon, an alkaline-earth metal, for instance magnesium, or a siliconfree alkaline-earth metal master alloy, is used as deoxidation agent, whereby the disadvantageous properties of the silicon in the alloy are avoided. Alloys deoxidized with magnesium, as an example, show much more sluggish precipitates. Moreover, these cover the grain boundaries only very slowly.

Therefore, and this is the special and unexpected advantage of the use of the alloys produced according to the invention, heat treatment is no longer necessary after the welding of sheets with a wall thickness up to e.g. 10 mm., in order to obtain a structure with a high corrosion resistance, especially to intergranular corrosion.

'In this way it is possible to weld vessels of any size with- 'out the necessity of a heat treatment and without encountering the great diiculties of annealing and quenching. Moreover, solution annealing does not require as high temperatures with silicon-free alloys as with those containing silicon.

For instance, a commercial alloy containing 56.8% nickel, '15.8% chromium, 16.5% molybdenum, 3.4% tungsten, 5.2% iron, 0.95% manganese, 0.052% carbon and 0.61% silicon, requires a temperature of 1,220 C. An almost equally composed,

molybdenum 17.0%, iron 3.8%, tungsten 3.1%, manga- 4nese 0.85%, carbon 0.04% and silicon 0.01%), could 'be solution annealed at 1,12 0 C. and quenching with water was not necessary as cooling in air was sutiicient.

Instead of magnesium, a mixture of silicon-free calcium with silicon-free strontium or silicon-free barium can be used for deoxidation. Other deoXidation media are however not suitable. Aluminum has proved to be very unfavorable.

Of the silicon-free alloys investigated, the alloys with the following composition proved to be especially favorable: 55 to 60% nickel, 22 to 25% chromium, 14 to 17% molybdenum, iron 2%, manganese l%, silicon 0 to 0.19% and carbon 0.08%. The range of dangerous precipitates is in addition reduced by the increase of the chromium content as compared with alloys of normal composition Without silicon. They offer, therefore, increased safety to intergranular corrosion, especially after welding.

In the event solution annealing is necessary for some reason, this can be done preferably at 1,150 C. and does not require a water quench as is the case with conventional alloys. For instance, 3 mm. thick sheets of a silicon-free nickel-chromium-molybdenum alloy having the following composition: nickel 61.3%, chromium 22.6%, molybdenum 14.0%, iron 1.3%, manganese 0.84%, silicon 0.012%, carbon 0.04%, have shown no grain boundary segregation after welding and quenching in still air. In contrast to this, commercial sheets of a thickness of 3 mm. with the following analysis:

Nickel 56.4%, chromium 15.3%, molybdenum 16.1%, tungsten 3.4%, iron 5.2%, silicon 0.61%, manganese 0.95%, carbon 0.05%

Nickel 59.1%, chromium 16.6%, molybdenum 16.9%, iron 5.8%, manganese 0.9%, carbon 0.06%, silicon 0.58%

Table 1 In the tests the following alloys were compared:

Alloy Ni Cr Mo Fe Mn C Si Mg MeltA (a commercia1auoy) 56.3 15.1 13.3 5.3 0.30 0.05 0.61 eltv 59.4 24.1 14.1 1.4 0.85 0.03 0.04 0.11

As is apparent from FIGS. l to 3 of the attached drawing the resistance against general corrosion (uniform attack) after solution annealing of an alloy prolduced from Melt B was far greater than an alloy produced from Melt A. This is due to the increased content of chromium and reduced content of silicon in the alloy of Melt B.

The resistance of the alloys to intercrystalline corrosion rises rapidly as the Si-content of the alloy drops. As -is apparent from FIG. 4 of the drawing, the corrosion effect is negligible up to an Si-content of- 0.1%. Intercrystalline corrosion occurs as a result of the formation of precipitates in the grain boundaries, the speed of precipitation depending on the Si-content. Such precipitates are particularly pronounced in the heat-affected zones of the Welding seams. Therefore, these zones are particularly endangered due to their susceptibility to intercrystalline corrosion. The degree of their susceptibility -is dependent on the duration and intensity of heating.

The graph set out in FIG. 4 shows the dependency of the thickness of alloy sheeting on its Si-content, i.e., what thickness such sheeting must have to insure that no intercrystalline corrosion occurs when such sheeting is not heated after welding. As mentioned above the impairment is negligible with an Si-content of up to 0.1%, but becomes severe even with as low an Si-content as 0.2%. The graph indicates the maximum thickness up to which alloy sheeting can be welded without the Welding seam having to be heat-treated. The area between the curve and the ordinate pertains to sheeting which will be adequately resistant to intercrystalline corrosion without heat treatment. On the other hand, if the values for Si-content and thickness fall outside this area, this means that the sheeting must be subjected to a heat treatment after welding.

When the two alloys described above are welded it is found that the alloy produced from Melt A is severely affected at the edges of its welding seam and especially in the heat-affected zones. In contrast, the welding seam in the heated zones remains unaffected with respect to the alloy of Melt B. As the Si-content of the alloy is increased, the precipitation zone is shifted toward higher temperatures so that very high solution annealing temperatures are required. In the case of the alloy of Melt A an annealing temperature of 1,220 C. is required. Moreover, due to the high speed of precipitation which results as the Si-content is increased, a quench with water is necessary to insure that the critical temperature range will be rapidly passed. In contrast, alloys having a maximum Si-content of 0.1% only require a temperature of 1,150 C. for solution annealing and can be cooled 1n air.

The differences in the properties of the alloys which occur as a function of the Si-content of the alloy also is evident with respect to the hardness and impact strength of the alloys as is evident from Table 2 which shows that the alloys become more substantially brittle as the Sicontent is increased.

The specimens used for the impact tests described in Table 2 were pieces 4 by 3 by 27 mm.

Obviously many modifications and variations of the invention as hereinbefore set forth may be made without departing from the spirit and scope thereof, and therefore only such limitations should be imposed as are indicated in the appended claims.

We claim:

1. A nickel-chromiurrr-molybdenum alloy having high corrosion and heat resistance properties, said alloy consisting essentially of from 40 to 65 nickel; up to 20% cobalt, the total amount of nickel and cobalt being from 40 to 65%; from 14 to 26% chromium; from 3 to 18% molybdenum; up to 30% iron; up to 5% tungsten; up to 0.1% carbon; up to 3% manganese; and a combined phosphorus and sulfur content of up to 0.1%, said alloy containing less than 0.2% silicon.

2. A nickel-chromium-molybdenum alloy having high corrosion and heat resistance properties, said alloy consisting essentially of from 55 to 60% nickel; up to 20% cobalt, the total amount of nickel and cobalt being from 55 to 60%; from 22 to 25% chromium; from 14 to 17% molybdenum; up to 7% iron; up to 5% tungsten; up to 0.1% carbon; up to 3% manganese; a combined phos- 65 Q3 phorus and sulfur content of up to 0.1%; and up to 0.1% molybdenum, about 1% iron, about 1% manganese, a silicon. maximum of 0.03% carbon, a maximum of 0.04% silicon,

3. A nickel-chromium-molybdenum alloy having high balll nickel.

corrosion and heat resistance properties, said alloy consisting essentially of from 55 to 60% nickel; from 22 to 25% 5 Referegs Cmd by the Examner chromium; from 14 to 17% molybdenum; up to 2% iron; UNITED STATES PATENTS up to 0.08% Carbon; up t0 1% manganese; and up t0 2,840,469 6/58 Gresham etal 75-171 0.012% silicon. 2,959,480 11/60 Flint 75-171 4. A nickel-chromium-molybdenum ailoy having high corrosion and heat resistance properties, said alloy con- 10 DAVID L RECK, PIWUY Examinersisting essentially of about 24% chromium, about 14% HYLAND BZQT, Examiner.

Claims

1. A NICKEL-CHROMIUN-MOLYBDENUM ALLOY HAVING HIGH CORROSION AND HEAT RESISTANCE PROPERTIES, SAID ALLOY CONSISTING ESSENTIALLY OF FROM 40 TO 65% NICKEL; UP TO 20% COBALT, THE TOTAL AMOUNT OF NICKEL AND COBALT BEING FROM 40 TO 65%; FROM 14 TO 26% CHROMIUM; FROM 3 TO 18% MOLYBDENUM; UP TO 30% IRON: UP TO 5% TUNGSTEN; UP TO 0.1% CARBON; UP TO 3% MANGANESE; AND A COMBINED PHOSPHORUS AND SULFUR CONTENT OF UP TO 0.1%, SAID ALLOY CONTAINING LESS THAN 0.2% SILICON.